1. Field of Invention
The present invention relates to devices for use in optical communication systems and, more particularly, to dispersion compensating devices for optical communication systems.
2. Discussion of Related Art
Wavelength division multiplexed optical communication systems obtain an increased signal-carrying capacity of an optical fiber by transmitting signals at several closely spaced wavelength channels. The wavelength channels must be confined within a limited wavelength band due to a number of constraints such as the transmission loss, the dispersion of the fibers and the gain characteristics of optical amplifiers. Currently, substantial efforts are being devoted to increase the number of channels in each optical fiber to increase signal-carrying capacity without having to install additional optical fibers. This requires crowding the optical channels closer and closer together as more wavelength channels are added. As the channel spacing decreases, chromatic dispersion of the signals becomes an increasing problem.
There are two primary approaches currently used to deal with the dispersion problem in wavelength division multiplexed (WDM) or dense WDM communication systems. The dispersion of an optical fiber depends on both the materials used in the fiber and the “structural” features of the optical fiber. In one approach, a fiber is produced to have a negative dispersion to compensate and/or cancel the dispersion at a given point in an optical transmission line. In the second approach, a chirped Bragg-grating optical fiber is produced to cancel and/or compensate for the dispersion at a given point in an optical communication line. However, both approaches require one to know the amount of dispersion in the given point in the communication line a priori to fabricate the compensating optical fiber. Optical communication systems that are currently installed are undergoing modifications to pieces of the system without the whole system being replaced. This can result in a change to the amount of dispersion at the location of the dispersion compensating fibers. Other factors that can cause the chromatic dispersion to vary in time would be temperature changes during the day, and on a longer time scale from session to session. Prior approaches would have to remove and/or add more dispersion compensating fibers to modify the systems, but they are limited to adapting to only slow time varying systems and not to more dynamic (i.e. quick varying) time changes.
Two of the current inventors previously produced a tunable dispersion compensating device in which a Bragg-grating fiber was embedded in a compliant material parallel to a load-bearing surface. In that device, a non-linear load is applied by imposing different forces along different portions of the load bearing surface to result in a chirped Bragg-grating within the fiber. S. T. Vohra, I. N. Duling, C. C. Chang, W. I. Kaechele, M. L. Dennis, and K. P. Koo, Proceedings of European Conf. on Optical Communications, Munich, Germany, September 2000. (The entire contents of this publication are incorporated herein by reference.) However, this proposed device requires applying different forces at different points along the load-bearing surface in a coordinated way to produce the desired chirp in the Bragg-grating fiber. The current invention is directed to improvements over such a proposed device.